The invention relates generally to systems for metering pumped fluids. It pertains more particularly to systems for measuring the flow of liquid asphalt or other substances in a bituminous pavement drum mixer.
The asphalt content of a bituminous pavement material produced in a drum mixer must be held within very close tolerances. Typical specifications might call for an asphalt content of 4.95% plus or minus 0.20%. A precision meter is normally used to accurately measure the amount of liquid asphalt which is pumped from the liquid asphalt reservoir and deposited into the drum mixer. A feedback system normally uses a computer to read the amount measured by the meter and, if too high, to automatically decrease the pump delivery or, if too low, increase the pump delivery.
In the past, however, precision meters have occasionally become inaccurate or have failed altogether in operation. Some failures can result in total loss of signal, while others may result in low output or erroneous signals. Also, excessive leakage past the metering elements will cause a low output measurement. Partial plugging can cause high differential pressure which also will result in a low output reading. Total plugging, or jamming of the metering elements will cause total loss of signal. Slippage of a gear or coupling on a shaft can cause either partial or total loss of output signal.
While present control systems call for total shutdown of the process with a total loss of meter signal, partial loss of signal will not shut down the control systems and related processes and will not otherwise alert the operators that either too much or too little liquid asphalt is going into the mix. Such a deviation in mix content can cause total loss of the asphalt mix, environmental damage or damage to the drum mixer.
Prior systems already utilize relatively expensive equipment such as a precision meter and computer for reading the meter and speeding up or slowing down the pump based on the feedback from the meter. However, there was no way of introducing redundancy of the meter reading to detect inaccuracies in the meter or partial signal errors without supplying an additional precision meter, which would be another piece of expensive equipment. Even with two precision meters, both meters could fail in the same way or for the same reasons at about the same time and therefore not warn of the gradually increasing inaccuracy of the other, back-up meter.
It is an object of the present invention to provide a system for inexpensively adding redundancy to a meter in a fluid conduit having a separate pump and meter.
It is another object of the present invention to provide an inexpensive system to back-up a precision meter of a fluid control system.
It is yet another object of the present invention to provide a computer controlled system for detecting inaccuracies in a precision meter.
It is yet another object of the present invention to provide a system to assure the meeting of narrow tolerances for the delivery of liquid asphalt or other fluids to a drum mixer.
It is yet a further object of the present invention to provide an inexpensive method to add redundancy to a meter in a system using a pump to flow fluid through the meter.
It is yet an additional object of the present invention to provide a method to inexpensively check for failure or errors in a precision metering of fluids in a bituminous pavement drum mix plant.
Other and further objects, features and advantages of this invention will be apparent from the following description and illustrations of the preferred and alternative embodiments of this invention.